Business Standard -- Ranbaxy Pharmaceuticals Inc had applied for marketing approval of a version of Purdue’s painkiller, Oxycodone. US-based Purdue Pharma has filed a patent infringement suit against India’s largest drug company, Ranbaxy, after the latter’s wholly-owned US subsidiary, Ranbaxy Pharmaceuticals Inc, applied for marketing approval of a low-cost version of Purdue’s pain relieving medicine, Oxycodone.